My understanding is that crankcase heaters in compressors are used to prevent the refrigerant from migrating to the oil due to low temperatures. This would cause increased foaming and loss of oil viscosity on startup.
It has also been stated that mineral oil will separate out and will pool up in the evaporator core if used with R-134a. This is due to a decrease in miscibility caused by decreased temperatures in the evaporator core. Just as in a cold compressor crankcase, it would seem that refrigerant-to-oil miscibility would increase in the evaporator core due to the cooler temps, just like in a cold compressor crankcase.